Man, the Tapus are made for VGC. Sorry for the long post and any inaccuracies, I hope you'll find this post interesting!
So, they are pretty special. I'm not completely sure if they count as Legendary Pokemon or not, but based on the leaks, it seems like they're right after the Jangmo-o line on the Pokedex. I'm not certain if this is accurate, but in any case it's apparent that they're not some regular Pokemon. They're constantly referred to as deities and their descriptions certainly match to something legendary. My point is, they'll likely have a diverse movepool and some very good stats, assuming they're pseudo or something like the trios we've met before (Legendary Birds, Dogs, Pixies etc.), and I'm 100% sure they won't get banned. A likely base stat total of 580 is great, especially in a format that won't allow Megas for some time.
The above is mostly hypothetical, but the real reason they stand out has already been confirmed. Really, really good type combinations and abilities. All of them share the secondary Fairy-type, undoubtedly one of the best types on the game, but their first types aren't bad either.
Code:
[B]Tapu Bulu (Grass/Fairy): [/B]
x4 Weak: Poison
x2 Weak: Fire, Ice, Flying, Steel
x0.5 Resists: Water, Electric, Grass, Fighting, Ground, Dark
Immunity: Dragon
[B]Tapu Lele (Psychic/Fairy): [/B]
x2 Weak: Poison, Ghost, Steel
x0.5 Resists: Psychic
x0.25 Resists: Fighting
Immunity: Dragon
[B]Tapu Fini (Water/Fairy):[/B]
x2 Weak: Electric, Grass, Poison
x0.5 Resists: Fire, Water, Ice, Fighting, Bug, Dark
Immunity: Dragon
[B]Tapu Koko (Electric/Fairy):[/B]
x2 Weak: Poison, Ground
x0.5 Resists: Electric, Flying, Fighting, Bug, Dark
Immunity: Dragon
As you can see they're pretty good types. Their major weakness seems to be poison, other than that it's quite balanced. Stuff like Dragon and Fighting, both of them are really common, just doesn't stand a chance. Water/Fairy is already known as a godlike combo that helped Pokemon like Azumarill ascend to glory, but Electric/Fairy is also really good that was wasted on Dedenne. The other two aren't particularly bad, but I see it as a step down compared to Koko's and Fini's typing. Bulu has quite a few weaknesses, but a significant number of resistances, while Lele gets normal damage from the majority of the types. Overall, really good types though and if a team has something to cover for the Poison weakness, I could even see teams with more than one of the Tapu.
For me though, their real strength is their abilities. All of them are variations of the same thing. They bring a Terrain for five turns when they enter the battle. First, five turns are plenty for the VGC format. Secondly, for people that aren't aware of Terrains, and I don't really blame them, they're a special room condition that changes the battlefield, similar to weather, though it doesn't count as a weather and it can be used along with it. Previously, they were locked on moves and, in addition, only a handful of Pokemon could use them, so they weren't really that popular. In Sun and Moon, they're the signature ability of the Tapus, that activates when they enter the battle, so there's not a turn that goes to waste and you can immediately take advantage of their effect. It has to be noted though that they effect only Pokemon on the ground, for better or worse.
Regarding their abilities, from the official site:
Tapu Bulus Ability is Grassy Surge, an Ability that no Pokémon has previously had. With Grassy Surge, the field will be turned into Grassy Terrain when Tapu Bulu appears in battle, boosting the power of any Grass-type moves used by Pokémon on the ground and restoring a bit of HP to them each turn. It also halves the damage taken from the moves Earthquake, Magnitude, and Bulldoze.
Tapu Leles Ability is Psychic Surge, an Ability that no Pokémon has previously had. With Psychic Surge, the field will be turned into Psychic Terrain when Tapu Lele appears in battle, boosting the power of any Psychic-type moves used by Pokémon on the ground and preventing them from being hit by priority moves.
Tapu Finis Ability is Misty Surge, an Ability that no Pokémon has previously had. With Misty Surge, the field will be turned into Misty Terrain when Tapu Fini appears in battle, halving the damage taken by Pokémon on the ground from Dragon-type moves and preventing them from being afflicted with status conditions or confusion.
Tapu Kokos Ability is Electric Surge, an Ability never before seen in a Pokémon. Electric Surge fills the field around the Pokémons feet with electricity when it appears, raising the power of any Electric-type moves for Pokémon on the ground and preventing any Pokémon from falling asleep.
Individually, every ability is great and it can be used according to the team set up or the situation. Electric Terrain's second effect is a bit disappointing though, as Fini already covers that. Bulu certainly covers a lot of ground, as its ability heals, empowers Grass type moves and depowers one of the most overused moves, Earthquake. Lele's is also really interesting, seems like a neat counter to moves like Bullet Punch or even Talonflame.
I really like Fini's ability though, as it essentially means no Burn or Paralysis. For example, it seems to work decently with one of my favourites, Garchomp, which would get crippled by a Burn. In a normal scenario, if the opposing Pokemon is on the ground, the dragon attacks are a bit useless, but there's still its devastating Earthquake. On the other hand, if the Pokemon is in the air, Dragon moves or something like Rock Slide work good, as usually. And this is just an example off the top of my head, I'm sure other Pokemon can benefit more from it.
I should also mention that the Tapu have their own signature move too, Nature's Madness. Its type isn't confirmed as far as I know, but its effect is pretty standard, it reduces the opponent's HP by half. If the Tapu is primarily offensive this doesn't seem that great, but for something like Fini that will likely have a supporting role, this is a god send. Paired with the right Pokemon, this is actual very helpful. I suppose that Lele and maybe Bulu can also work as supporting Pokemon, but it's kind of baseless. In any case, it's not a bad move, but I'm sure their movepool will have a lot too offer anyway, so it might end up as a gimmick. There's also Aurora Veil, a Fairy type move that acts as a Reflect
and a Barrier, it's not currently known which Pokemon can learn it, but if any of the Tapus can, it seals the deal.
Anyway, I hope I made it clear why Tapus have a lot of potential. Of course, I'm not taking into account a lot of factors, but I think I highlighted the basic on why I think they'll be very prominent. Tapus are a very interesting addition to the Pokemon Universe and they really seem to be exactly made for the VGC. Even if they're non breedable and catchable only once, the Hyper Training stuff that will be introduced in Sun/Moon, means that it's possible to make them perfect and that's very important for them to be viable. If weather conditions and Mega Pokemon were the face of the Gen V and VI competitive scene respectively, I think the Tapus and their Terrain abilities will be the face of Gen VII.
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